Conventionally, an electromagnetic antenna is composed of a radiating element, a dielectric and a ground plane. The radiating element and the ground plane are most commonly metal. They are of very diverse shapes and dimensions.
In radio systems of the software radio type, such as SDR (for Software Defined Radio in English) or SR (for Software Radio), the terminals and/or communicating objects are limited in size and weight and have a poor energy autonomy. These terminals and/or communicating objects require antennas which are miniaturized, on the one hand, and that, on the other, can satisfy a set of constraints associated with the radio system. For example, these antennas must be able to simultaneously cover all the frequencies of a wide frequency band or, as a minimum, these antennas must be very flexible in frequency in order to be able to scan a wide spectrum of frequencies.
In order to satisfy this set of constraints, antennas referred to as “reconfigurable” have been designed.
At least three types of reconfigurable antennas are currently available as presented hereinbelow.
Antenna Reconfigurable in Frequency:
The antenna is then called frequency-flexible. The antenna can thus scan a wide spectrum of frequencies. Such antennas are used in mobile terminals which can be compatible with several communications standards such as the GSM standard (for Global System for Mobile Communications in English) which relates to a frequency band around 900 MHz and the UMTS standard (for Universal Mobile Telecommunications System in English) which relates to a frequency band around 1800 MHz.
Antenna Reconfigurable in Polarization:
The antenna is then referred to as polarization-flexible. For a linear polarization, this polarization may be horizontal or vertical, and for a circular polarization, this may be left or right. Such antennas provide a better signal-to-noise ratio and are particularly advantageous in locations where the propagation of electromagnetic waves encounters numerous obstacles, such as for example inside buildings.
Antenna Reconfigurable in Radiation Pattern:
The antenna is then capable of modifying its radiation pattern in order, for example, to adapt to a change in the propagation environment.
In the current prior art, in view of the dimensional constraints, the reconfiguration of an antenna is not achieved by a mechanical or geometric deformation of the antenna or of the elements composing it.
In fact, the reconfiguration of an antenna is currently achieved by switching certain elements within the radiating element, the dielectric and the ground plane that compose it, or by varying impedances connected to certain points on the antenna.
These two modes of reconfiguration present certain drawbacks.
In the case where elements of the antenna are switched, a discontinuous variation of the characteristics that it is desired to reconfigure (frequency, directivity of the radiation) is obtained.
In the case where impedances connected to certain points on the antenna are varied, a continuous variation in frequency is obtained but limited by the ranges of variation of the impedances used. For the same reasons, the continuous variations of the radiation pattern are limited.
The combination of the two types of reconfiguration (by switching of elements and by variation of impedances) allows variations of the physical characteristics in question to be obtained over wider ranges but with an increased complexity that tends to be incompatible with the design constraints (dimensions, weight, energy autonomy) of the terminals and/or the communicating objects in question.
Furthermore, the elements (switches and impedances) enabling the reconfiguration exhibit intrinsic losses which affect the efficiency of the antenna.
There is therefore a real need for a technique for reconfiguring an electromagnetic antenna which does not exhibit the aforementioned drawbacks of the known reconfiguration techniques.